Conventional voice coil assemblies manufactured by way of photochemical etching processes, comprises a nonconductive substrate and a metallic spiral pattern layer made on the substrate. The metallic spiral pattern layer functions as a voice coil and the substrate having the voice coil is supported by a suitable holder at the periphery of the substrate, while permanent magnets are located in the vicinity of the coil so that the substrate vibrates when the coil is energized by an input signal.
As is well known, the voice coil tends to generate heat when energized. The degree of this heat generation is a function of the power of the input signal. Since the voice coil is made from a thin microstripline, a high temperature may cause the voice coil to burn out. Therefore, the allowable maximum input power is simply determined by the resistivity of the coil and the efficiency of the heat dissipation. The resistivity of the coil is determined by the thickness and the width of the coil and both the thickness and the width thereof cannot be increased for the following reasons.
A predetermined number of turns has to be made within a limited space on the substrate. Accordingly the increase in the width of the coil results in either the increase in the dimensions of the voice coil assembly or the reduction of the number of turns. The increase in the thickness of the coil results in the increase in weight of the coil assembly deteriorating the efficiency of sound generation especially in a high frequency range. From the above, it is concluded that it is impossible to lower the resistivity of the coil.
Consequently, the only possible way to increase the allowable input maximum power is to increase the efficiency of the heat dissipation. However, according to the prior art the heat generated by the voice coil is dissipated only through the substrate the thermal conductivity of which is relatively low.
Furthermore, according to the conventional technique for manufacturing voice coil assemblies by way of photochemical etching processes, two terminals of the voice coil are respectively connected to two input external terminals of the voice coil assembly by means of conductive lead wires. This means that two lead wires have to be bonded to respective terminals at four places. The lead wires have possibilities of being broken, while the lead wires might be poorly soldered at the respective terminals.
In addition to the above described disadvantages of the conventional voice coil assembly, the frequency characteristic of the conventional speaker of the type above mentioned is not flat because of the existence of standing waves across the voice coil. These standing waves are generated by a holder which supports the substrate when the holder has a central opening defined by two pairs of equidistantly spaced sides.